如何在dart中获得子类的属性?

Question

我有一个类a和一些属性:

abstract class A {
double doubleA;
String stringA;
...
A({this.doubleA = 0, this.stringA = ""});
}

和具有某些属性的类B，它扩展了类a:

class B extends A {
int intB;
String stringB;
B({
this.intB = 0, 
this.stringB = "",
double doubleA = 0, 
String stringA = "",
}) : super(doubleA: doubleA, stringA: stringA);
}

在我的代码中，我现在想检查A的实例是否具有子类B类型的值:

A a; // Value can be of different subtypes of A including B
if(a is B) {
// here dart should give me access to the properties of a like:
print(a.stringA);
// but it should also be possible to access the type B properties 
// since the value of a can also be of subclass type B:
print(a.stringB);
}

这听起来不对，但我知道它可以工作，因为在扑动的例子。

示例侦听器:

Listener(
onPointerSignal: (event) {
// event is of type PointerSignalEvent which has no property 'scrollDelta'.
// So print(event.scrollData); does not work here.
if (event is PointerScrollEvent) {
// if you check if event is of subtype PointerScrollEvent the property 'scrollDelta'
// that is included in the class PointerScrollEvent becomes available.
print(event.scrollDelta); // works without any problem.
}
},
}

然而，我没有能够复制这与我的类A和B，我不知道为什么它不起作用。我还研究了这些颤振类的实现并复制了类结构，但我仍然只能在检查if(a is B)后访问A的属性，这与颤振类观察到的行为不一致。

我做错了什么?我错过了什么吗?

感谢阅读:D <3

Answer 1

如果将变量'a'声明为'a'类型，则无法访问继承自a的类(如B)的属性。假设A是动物，B是狒狒。Baboon继承了Animal的属性，所以所有用Baboon类型实例化的变量都可以访问这两个类的属性。但是用Animal类型实例化的变量只能访问Animal属性。下面是一些例子:https://medium.com/jay-tillu/inheritance-in-dart-bd0895883265